1. Field of the Invention
This invention is generally related to a shield structure, and more particularly to a shield structure in which a shield case shields a first circuitry block mounted on a first circuit board and a second circuitry block mounted on a second circuit board in an electric apparatus.
2. Description of the Related Art
An electronic apparatus includes electronic components which often generate electromagnetic signals. The signals may radiate and interfere with the operation of other components. Further, the electronic apparatus may receive electromagnetic signals which are generated by other components and may cause interference with its own internal operation. To minimize interference, an electrically conductive case generally is provided to shield the electronic components from other components.
Further, an electric apparatus such as a portable apparatus often includes a plurality of circuit boards to provide many components in the electric apparatus. For ease of explanation, components on a first circuit board are referred to as a first circuitry block hereinafter, and components on a second circuit board are referred to as a second circuitry block hereinafter. If the first circuitry block includes components which may generate electromagnetic signals, the first circuitry block is required to be covered by a conductive shield case. Likewise, if the second circuitry block includes components which may generate electromagnetic signals, the second circuitry block is required to be covered by a conductive shield case.
In order to decrease the size and weight of the electronic apparatus, the first and second circuitry blocks are often covered by only one conductive material having two partitioned spaces, instead of two conductive shield cases. Accordingly, the electronic apparatus becomes light and small. An example of such a conventional structure is shown in FIGS. 5 and 6. Referring to FIG. 5, an electronic apparatus 1 comprises a housing 2, a first circuit board 4, a second circuit board 6, and a shield case 8 made of aluminum. Components 8a and 8b constituting a first circuitry block 10 are mounted on the first circuit board 4. Components 12a, 12b, and 12c constituting a second circuitry block 14 are mounted on the second circuit board 6. Further, a first ground 16 is provided on the first circuit board 4. A second ground 18 is provided on the second circuit board 6. The shield case 8 comprises side walls 20 and a partition wall 22 to divide the space within the shield case 8 into two sections. When the shield case 8 is attached to the first and second circuit boards 4, 6, the first and second circuitry blocks 10, 14are shielded against other components.
To enhance the connection state between the shield case 8 and the grounds 16, 18, as shown in FIG. 6, conductive braid materials 24, 25 having an elastic nature are provided between the shield case 8 and the circuit boards 4, 6. Specifically, the wall 20 has grooves 26, 28 at opposite end faces 30, 32 for supporting the conductive braid materials 24, 25. When the conductive braid materials 24, 25 are engaged with the grooves 26, 28, and the first circuit board 4 and the second circuit board 6 are attached to the shield case 8, the shield case 8 is sufficiently coupled to the ground 16, 18 due to the elastic nature of the conductive braid materials 24, 25.
However, the grooves 26, 28 are provided in each of the end faces 30, 32. Therefore, the total area of the end faces 30, 32 is relatively large. As a result, the space in each of the circuit boards 4, 6 occupied by the wall 20 must be increased. Further, the size of the shield cases 8 similarly becomes relatively large.
Since electronic apparatus such as portable radio telephone apparatus are increasingly becoming miniaturized, the area in the circuit boards 4,6 required for the end faces of the shield case 8 is increasingly becoming a design constraint which limits further miniaturization of the electronic apparatus. Further, the size of the shield cases 8 similarly is also increasingly becoming a design constraint which limits further miniaturization of the electronic apparatus.
Further, the attachment of the conductive braid materials 24, 25 to the grooves 26, 28 must be performed along the entire end faces having the grooves 26, 28. Therefore, it is inconvenient.
Furthermore, the shield case 8 is electrically coupled to the grounds 16, 18 by the two conductive braid materials 24, 25. As a result, four contact portions exist between the grounds 16, 18. A portion between the ground 16 and the ground 18 has a contact resistance corresponding to the four contact portions. Since the portion covering the components has a relatively high resistance, the portion can function as an antenna which can receive electromagnetic signals transmitted by the other components outside of the shield case 8. This tendency is well known in the prior art. As a result, the portion can radiate the received electromagnetic signals to the first and second circuitry blocks 10, 14 in the shield case 8. These radiated electromagnetic signals can interfere with the normal operation of the first and second circuitry blocks 10, 14.